Devices and methods for copying of documents are well known in the prior art. Included in such prior art are devices and methods for enhancing the optical recognition of characters in documents being copied, and such character recognition devices have commonly included threshold circuitry. Such devices and method are shown, for example, in U.S. Pat. Nos. 3,479,642, 3,534,334 and 3,737,855.
It is also well known in the prior art to provide switching between different modes in a facsimile system, and this is shown, for example, in U.S. Pat. Nos. 3,294,896, 3,622,698 and 3,723,649.
In connection with facsimile systems, it has also been heretofore suggested that switching can be accomplished between two conditions depending upon whether the portions of a particular document then being copied consists of half-tones or black-and-white matter. A two condition thresholding arrangement is shown, for example, in U.S. Pat. No. 2,255,408, with circuitry being disclosed for switching between line copy and continuous tone detection algorithms based upon the rate of change of reflectance from the portion of the document then being copied. It is also suggested in U.S. Pat. No. 2,255,408 that unnecessary switching can be avoided by averaging the area over which the gradient change is selected.
A multiple threshold detector system having three range decision circuitry is also shown in the prior art (see, for example, U.S. Pat. No. 3,723,649), but the circuitry such as shown in U.S. Pat. No. 3,723,649 necessarily includes hysteresis that is operative in the decision logic to provide a change in the binary signal level indicative of a threshold level opposite to that establishing the then existing binary level to accomplish the stated end.
Thus, while the prior art shows thresholding mode switching circuits and methods for mode selection, the circuits and methods described are either capable of switching only between two conditions or necessarily require the use of hysteresis where more than two decisions are required.